1. Field of the Invention
The present invention relates to a method and an apparatus for accurately detecting a frequency of an AC electric parameter of an electric power system.
2. Description of the Related Art
In an electric power system including electric power plants for generating power, transfer plants, such as transmission lines and substations, for transferring the power generated by the electric power plants, and demand plants which consume the power transferred and distributed by the transfer plants, when a generator of an electric power plant is stopped, since power supplied to the electric power system is reduced, loads on functioning generators are increased and the rotation rates of the generators are lowered. In contrast, when a load, i.e., a demand plant, is eliminated, since power is oversupplied to the electric power system, the generators of the electric power plants are accelerated and the frequency of the electric power system is increased.
An electric power system is most stabilized when the frequency thereof corresponds to a fundamental frequency. Therefore, in order that the electric power system is operated stably, it is necessary to detect, with high accuracy, how much the frequency of the electric power system is deviated from the fundamental frequency. Further, in controlling excitation of a generator, it is necessary to detect a frequency in a wide range at high speed.
In consideration of the aforementioned technical background, a method for detecting how much the frequency of the electric power system is deviated from the fundamental frequency, using the following algorithm, is proposed. ##EQU1##
In the above formula, "m" represents a sampling time series and a sampling frequency is 12 times the fundamental frequency of an AC electric parameter of the electric power system. Thus, the time series (m-3) means data which is 1/4 cycle late for the time series m. ".epsilon." represents a relative error with respect to the fundamental frequency of 1. The relative error ".epsilon." is obtained by the following equation: .epsilon.=.DELTA.f/fo=.DELTA..omega./.omega.o, where the fundamental frequency of the electric power system is fo (.omega.o) and a deviation from the fundamental frequency fo (.omega.o) is .DELTA.f(f-f0).
However, as clear from FIG. 8 showing a frequency characteristic, the linearity of the frequency characteristic is decreased and a calculation error respect to a left side of the formula (1a) is increased, in accordance with the increase of the relative error .epsilon. (quadrature axis), i.e., the deviation between the frequency f of the electric power system and the fundamental frequency f0. Thus, a relative error .epsilon. cannot be calculated accurately. Therefore, in a conventional method, the formula (1a) is preset in a known table to detect the relative error .epsilon.. A calculation result of the left side of the formula (1a) is input to the table, thereby obtaining a relative error .epsilon. output in accordance with the calculation result.
Therefore, a frequency can be detected accurately only within a narrow range in which the relative error .epsilon. is nearly zero and the linearity of the frequency characteristic in FIG. 8 is maintained, i.e., a range of the frequency nearly the fundamental frequency of the AC electric parameter in FIG. 8.